Wash and cut tomatoes in big pieces. Cut the onion in big pieces too. In a bowl add the tomatoes, onions, garlic cloves and black pepper corns. Add a cup of water and boil till the tomatoes are cooked. Alternatively microwave the same for about 4 to 5 minutes.Then strain the above boiled mixture in a bowl through a strainer. Add water as required while straining so that all the pulp is strained nicely. You will get nice thin pulp of all the boiled tomatoes, onions and garlic .Keep the bowl on medium gas, to that add red chilli powder, turmeric powder, rassam powder, sugar / jaggery as per your choice and salt to taste. And let it boil and simmer it for couple of minutes and switch off the stove.For tadka / tampering :Heat oil / ghee as per choice in a small pan to it add mustard seeds, pinch of heeng, dry red chillies cut into pieces and curry leaves. Once these splutter add the tampering to the above cooked rassam/ saar . Garnish with fresh coriander leaves and the tomatoe saar or rassam is ready to serve with Rice. Or have it like soup too.